State Route 139 (SR 139) is a 59-mile-long (95 km) state highway traveling from U.S. Route 2 (US 2) and US 201A in Norridgewock at its western terminus to US 1A in Winterport at its eastern terminus.

The section of SR 139 between U.S. Route 2 in Norridgewock and Interstate 95 in Fairfield is often used as a bypass of U.S Route 2 through Skowhegan.

From the interchange with Interstate 95, as it heads east there is a 0.22 concurrency with U.S. Route 201 in Fairfield and then a 2.23 mile concurrency with SR 11 and SR 100 in Benton.

SR 139 continues on to have a 0.18 mile concurrency with U.S. Route 202 and SR 9 before once more being without concurrent routes for 0.69 miles in Unity and then begins a 3.5 mile concurrency with SR 220 in Thorndike.

From there, SR 139 has a 1.67 mile concurrency with SR 69 and reaches its western terminus at US Route 1A in Winterport.